There are two commonly used methods of mechanically mining coal underground. The first of these is generally known as longwall mining, and involves removing substantially all the coal from a seam and allowing the strata above the seam to collapse once the coal has been extracted. This method is suitable for relatively thin seams at relatively large depths, since the effect of allowing the strata to collapse is generally not noticeable on the surface. However, this method is not suitable for thicker seams or seams at smaller depths.
For such seams, the second method, generally known as room-and-pillar mining is suitable. In this method a substantial proportion of the coal in the seam is not removed but is left as pillars surrounded by areas (rooms) from which the coal has been removed. The pillars serve to support the above-lying strata and prevent subsidence occurring at the surface.
Room-and-pillar mining is generally carried out using machines known as continuous miners. A continuous miner generally comprises a movable chassis, for instance mounted on caterpillar tracks.
A pivot mounting is formed integrally on the upper surface of the chassis, for instance as a casting or machined part. A cutting boom extending parallel to the longitudinal axis of the chassis is pivotally mounted on the pivot mounting for movement in a vertical plane by the action of hydraulic rams. The rams are generally connected between the boom and connection points formed integrally on the chassis.
The cutting boom carrier at its free end a generally cylindrical cutting head whose cylindrical axis is arranged horizontally. The cutting head is somewhat wider than the width of the chassis. Common widths for the cutting head are, two, three or four meters. The cutting head is provided with a plurality, for instance about fifty, of cutting picks disposed circumferentially around and extending radially from the cutting head. The cutting head is rotatable about its axis, generally by electric motors and chain drives located on the cutting boom.
A coal clearing means, generally comprising a gathering apron having wing plates at its edges, is provided on the chassis for removing coal cut by the continuous miner from below the cutting head to the rear of the machine. The cut coal is generally forced onto the clearing means by the forward movement of the machine.
In use of the continuous miner, the cutting head is set in rotation, and the machine is moved towards the coal to be cut. As the picks contact the coal, it is ripped out of the seam and falls onto the floor in front of the machine. Once the cutting head has penetrated a given distance into the seam, the boom is pivoted through its fullest possible range to cut out a panel of coal. Thereafter, the machine is again moved towards the coal seam and the operation is repeated. During forward movement of the machine, the cut coal is loaded onto the coal clearing means. Thus, during its operation, the continuous miner forms a tunnel in the coal down which it travels to cut more coal.
A disadvantage of such continuous miners, which has been known for a considerable time, is that they are unable to cut material having a compressive strength of more than about 10,000 psi (68.9 MPa) due to the very large numbers of picks on the head. It is not possible to supply sufficient power to the head to drive this number of picks through material having a compressive strength above the value given above.
Thus, if a continuous miner comes across an area of hard coal, an area in which the coal is interspersed with hard material, or an area of hard material, for instance a rock fault extending through the seam, its progress is halted.
There are generally two methods presently used for enabling the continuous miner to progress beyond the area of hard material. In the first method, the continuous miner is moved backwards until it is entirely out of the tunnel it has created. This may merely entail moving it into a stall created at the side of the tunnel, but may instead entail moving the continuous miner back to a main roadway or the pit bottom. In view of the amount of ancilliary equipment which is set up behind the continuous miner, this operation could take at least 10 shifts to complete.
Once the continuous miner has been removed from the tunnel, a roadheader is moved into the tunnel and used to cut through the area of hard material. Putting the roadheader in place generally takes at least 10 shifts to complete.
Roadheaders are well known in the art and can cut through material having a compressive strength of up to about 30,000 psi (206.8 MPa).
They generally comprise a chassis on which is mounted a turntable assembly including a turntable rotatable in the horizontal plane. The turntable is usually rotatably by use of hydraulic slewing rams connected between the turntable and the chassis. The turntable assembly includes a pivot mounting and a roadheading boom mounted for pivotal movement in a vertical plane on the pivot mounting. The roadheading boom is usually pivoted by means of one or more rams connected between the boom and the pivot mounting. The roadheading boom is arranged to carry a generally conical head having arranged on it in a scroll formation up to about twenty picks. The conical head is rotatable
about the longitudinal axis of the boom by means of motors and gearboxes in the boom. Once the area of hard material has been removed and the coal re-exposed, the roadheader is removed and the continuous miner is replaced in the tunnel to win the newly exposed coal. The processes of removing the roadheader and replacing the continuous miner generally takes at least ten shifts. There is thus a very large down time involved in this method even without counting the time needed for the roadheader to cut through the area of hard material.
The second method involves the use of drilling and blasting the area of hard material. If the hard material can be drilled by hand-held drills or drills which can be mounted on small tripods or the like, it will only be necessary to move the continuous miner back a safe distance from the area of hard material. It will, in this case, not be necessary to remove the continuous miner from the tunnel.
However, in some cases, it will be necessary to drill the hard material using a drilling machine. In this case it may be necessary to remove the continuous miner from the tunnel and move in a drilling machine, with all the disadvantages in terms of down time referred to above.
Although in many cases it is possible to begin a drilling and blasting operation more quickly than it would be possible to set up a roadheading operation, it is not necessarily quicker to remove the hard material by drilling and blasting. Once set up, a roadheading operation is generally much quicker than a drilling and blasting operation. It may therefore be overall time-effective to use the roadheading operation.
Moreover, in some cases, it may be environmentally unacceptable to use a drilling and blasting operation, for instance if the mining operation is being carried out under or near a community or an area which is particularly susceptible to seismic shocks. In such cases it will be necessary to carry out a roadheading operation.
Although this disadvantage of having a large amount of unproductive time during a room-and-piller mining operation when an area of hard material is encountered has been known for a considerable time, no proposals for overcoming this disadvantage have been made.
It is an aim of the present invention to provide a solution to the problem set out above.